Body coupled communications (BCC) or body-based communication has been proposed as a promising alternative to radio frequency (RF) communication as a basis for body area networks. BCC allows exchange of information between a plurality of devices which are at or in close proximity of a body of a human or an animal. This can be achieved by capacitive or galvanic coupling of low-energy electrical fields onto the body surface. Hence, the human body is exploited as a communication channel, so that communication can take place with much lower power consumption than in standard radio systems commonly used for body area networks (BANs, e.g. ZigBee or Bluetooth systems). Since BCC is usually applied in close proximity to the body, it can be used to realize new and intuitive body-device interfaces based on contact or proximity.
BCC can be technically realized by electric fields that are generated by a small body-worn tag, e.g., being integrated into a credit card or another suitable device attached to or worn in close proximity to the body. This tag capacitively or galavanicly couples a low-power signal to the body. Sometimes this body-coupled communication is referred to as “near-field intra-body communication”. BCC is a wireless technology that allows electronic devices on and near the human body to exchange digital information through capacitive or galvanic coupling. Information is transmitted by modulating electric fields and either capacitively or galvanicly coupling tiny currents in the range of pico amperes onto the body. The body conducts the tiny current (e.g., 50 pA) to body mounted receivers. The environment (the air and/or earth ground) provides a return path for the transmitted signal.
FIGS. 1A and 1B show a general transmitter-receiver model of capacitive BCC, as described in Thomas Guthrie Zimmerman, Personal Area Networks (PAN): Near-Field Intra-Body Communication. Master of Thesis, Massachusetts Institute of Technology, September 1995. Both TX and RX elements are electronic battery powered devices, electrically isolated and having a pair of electrodes A and B, acting as reference and signal electrode. The transmitter and receiver electrodes A, B can be modelled as capacitor plates. Such capacitive BCC can be used for data exchange between devices in so-called personal area networks (PANs) or body area networks (BANs). A PAN prototype working at 330 kHz was developed to demonstrate the digital exchange of data through the body. Examples of such communication systems are disclosed for example in U.S. Pat. No. 6,992,565, U.S. Pat. No. 6,777,992, U.S. Pat. No. 6,223,018, and U.S. Pat. No. 5,914,701.
The electrodes A, B can be made of common printed circuit board (PCB) material with copper surfaces. The size could be approximately 3 cm×5 cm×1 cm. Measurements have shown reliable communication over the whole human body, regardless of the location of both TX and RX devices.
Although the devices FIGS. 1A and 1B have good sensitivity and give a strong signal even when the TX and RX devices are far apart on the body, the orientation of the RX/TX devices with respect to the human body is critical and largely influences the communication performance. For capacitive BCC, ideally, the TX and RX devices are lying flat on the body surface, with one electrode in close contact to the body and the other having “free sight” to the surroundings.
This is the case since the performance of capacitive BCC is influenced by a capacitive coupling between the transmitter (TX) and receiver (RX) devices and the human body and all surrounding conductors. In this scenario the most critical aspect is represented by the orientation of the devices with respect to the body. The signal attenuation strongly depends on the ratio between the capacitive coupling to human body of the signal electrodes and reference electrodes. The closer the coupling values the weaker the received/transmitted signal. A solution to this problem is crucial in order to have reliable communication between BCC devices that are not fixed to the human body but that can be also loosely placed around it, as in most of the possible scenarios.
In FIG. 1A both TX and RX devices are in a flat angle with respect to the human body. Electrodes B have thus a much stronger capacitance C2b to the body than to the surroundings (capacitance C1b), while electrodes A are better coupled to the surroundings (capacitance C1a) and have only small coupling to the body (capacitance C2a). This way a signal that is applied between the TX electrodes will generate a voltage difference between the electrodes in the receiver, thereby enabling data transfer.
However, in FIG. 1B, the RX device is rotated by 90 degrees, which means that both electrodes A and B of the RX device have equal coupling to the body (capacitance C2a=C2b) and surroundings (capacitance C1a=C1b). As a result, any signal that is conducted by the body (as for instance a signal generated by the TX device) is common mode for electrodes A and B in the RX device. In other words, both electrodes A and B see the same potential and there is no voltage difference. The same holds for situations where the TX device is rotated by 90 degrees. In that case no signal is provided to the body and signal transfer is impossible. Of course, the 90 degree angle is a limit situation, but it will be clear that any other rotation (with respect to the ideal flat angle) will lead to a decrease of the received signal power.
In order to provide close contact and flat angle to the human body, it has been proposed to fix electrodes to the body (e.g. by means of stickers or elastic bands) but this puts a strong limit on the possible applications, since in most scenarios the TX/RX devices are expected to be only in proximity to the body and loosely coupled to it (e.g. any device that could placed in pocket, such as a mobile phone, or be integrated in textile). Moreover also in the case of devices that can be stitched to the human body (for example, medical sensors), movements of the user during normal life can affect the capacitance configuration of the transmitting/receiving devices in a way that reliable communication cannot be guaranteed.
Communications using BCC is thus sensitive to the orientation of the couplers relative to the body. Due to this sensitivity to the orientation of the couplers, the user is required to place and keep the BCC devices in a well-defined orientation. This creates user-inconvenience and unreliable transmission. Hence, it is desirable to make the BCC communication between devices which can be placed somewhere at the body reliable, so that the communication range for these devices is the whole body.